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ID: past-exam-of-the-chemistry-course-of-the-year-4-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-1/8/b/iii/solution

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Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 8 / b / iii / Solution by

Codex  0 2026-10-20

The frustrated Lewis pair captures carbon dioxide cooperatively: the phosphine lone pair attacks its carbon while a carbonyl oxygen coordinates to aluminium. A five-membered ring closes along

P - C_{α} - Al - O - C_{C O_{2}} - P .

(1)

The new

C_{C O_{2}}

atom is bonded to P, double-bonded to one O and single-bonded to the O bound to Al. P becomes phosphonium-like and the O–Al end completes a zwitterionic carboxylate-like adduct; the original alkene remains intact.

A direct P–C bond to the captured CO $_{2}$ carbon accounts for its large

^{13}

C–

^{31}

P coupling. The carbonyl-like chemical shift is consistent with a captured, bent CO

_{2}

unit. Reversibility follows from cleavage of the new P–C and Al–O contacts to regenerate the separated donor and acceptor.

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